The present invention relates to a sensor probe for determining the conductivity of a fluid. More particularly, the present invention relates to a non-fouling, open-cell four electrode conductivity sensor for oceanic use.
Conductivity sensors have previously been developed for use in monitoring oceanic environmental conditions and other research applications. For example, an array of sensors may be positioned along a cable towed by a surface ship so as to monitor environmental conditions of the water below the surface ships. Sensor arrays may also be towed by submarines. However, such prior art sensors have several drawbacks relating to measurement accuracy and fouling by biological matter, such as seaweed.
A typical example of the prior art is U.S. Pat. No. 3,601,693 to Lorentzen which discloses a four electrode closed-cell conductivity measuring device. The measuring cell includes a pair of current electrodes and a pair of voltage electrodes. The current electrodes are connected in series with an alternating current source and a current measuring device. The voltage electrodes are connected to a voltage measuring device. A ratio of the measured current flow to the measured voltage provides an indication of the electrolytic conductivity of the fluid contained in the measuring device.
U.S. Pat. No. 3,939,408 issued to Brown relates to a T-shaped four electrode closed-cell conductivity probe. The probe includes a pair of voltage electrodes and a pair of current electrodes. The voltage electrodes are connected in the input circuit of a high gain amplifier which has a negative feedback loop in which the current electrodes are connected. The feedback control circuit tends to maintain at all times the voltage at a value equal the reference voltage. The ratio of the amplitude of the feedback current and the AC reference signal is linearly proportional to the conductivity of the fluid within the cell.
Closed-cell probes of the type described above have inherent drawbacks relating to fouling, a high degree of flow distortion, poor flushing, large signal drift, poor dynamic response, relatively large dead band, cross-circulation, potential field complexity, and angle-of-attack sensitivity. Accordingly, these devices have not proven to be completely acceptable in practice.
Two electrode configurations have also been developed which have the advantage of an open-cell configuration. These devices, however, are relatively unstable, include a high electronic noise level that may interfere with neighboring instrumentation, are serious affected by biological material present in the fluid being tested, draw large electric currents, and are of questionable durability.
It has been necessary to provide an accurate and efficient conductivity probe, for use in oceanic application, which is not prone to fouling from biological materials. Accordingly, it is a primary object of the present invention to provide a highly accurate and efficient non-fouling conductivity probe which overcomes the foregoing disadvantages.